In the operation of a compressor, it must be ensured that the ratio of the final pressure to an intake pressure is not so high that the throughput through the compressor falls below a certain minimum amount. This lower limit is defined essentially by greatly increasing vibrations of the machine, which are caused inter alia by so-called surges. Those skilled in the art refer to the limit between regular operation and this so-called surging as the surge limit. This surge limit depends essentially on the ratio between the intake pressure and the final pressure. For the surge limitation of turbo compressors, algorithms that continually determine the margin from the surge limit on the basis of multiple measured values are used. If this margin becomes too small, the limiter opens a bypass valve, and thus ensures a minimum margin from the surge limit, at which the final pressure is lowered and accordingly the required throughput, mass flow or volume flow is restored. During normal operation unaffected by any fault, the so-called surge limitation for controlling the bypass valve protects the compressor from damage that would be caused by surging, it being necessary to ensure that the protection of the machine from this damage is still provided if there is a failure of one of the measuring signals involved.
One possibility of responding to a signal failure at the measuring points is to substitute the fault-affected measured value by the worst conceivable measured value—that is to say a value that gives a greater proximity to the surge limit then is the case in reality. The machine is consequently still protected from surges, but it may be that the bypass valve, which is usually formed as a control valve, is opened unnecessarily, with the result that the efficiency of the installation deteriorates. At worst, the valve opens so far that the throughput through the compressor is reduced to an unacceptable minimum, and so the connected process, which is generally strongly dependent on the compression process, can no longer be maintained.
Apart from the possibility described above, it is also known for the surge limiter to generate in the case of a fault a constant output signal that opens the valve to such an extent that under some circumstances surging may occur. This too leads to an adverse influence on the connected process.
The measured values that are measured generally also comprise a measurement of the throughput through the compressor. In this connection, it is also known in the event of a fault that does not concern the measurement of the throughput to adjust the compressor to the throughput of a minimum value at which surging is ruled out.
A further known possibility of countering a fault in the measurements is to specify that the pressure ratio is assumed to be constant and to correspond to the greatest possible pressure ratio to be expected, which likewise gives a great margin from the surge limit. A failure of the suction pressure measurement cannot be managed in this way.